In conventional discharge lamp ballasts, HID lamps are operated by, for example, square wave voltage. In case of an extra high pressure mercury lamp utilized as a light source for a projector, square wave voltage with a comparatively low frequency (approx. 100s Hz) is applied across the lamp in order to prevent occurrence of acoustic resonance phenomenon (see, e.g., Japanese Patent Application Publication number 2002-352982).
In these sorts of applications, the arc length of the lamp is required to be as short as possible in order to approximate it to a point source. However, if the arc length is shortened, an arc generation point on an electrode gets unstable depending on the temperature or surface condition of the electrode, so that the phenomenon that the arc origin point jumps to other points is easy to occur. When this sort of phenomenon occurs, visible flicker is perceived in the light output of the lamp, and in case that the lamp is utilized as a light source for a projector, problems are raised, such as difficulty in seeing a screen image due to brightness fluctuation or luminance reduction on plane of projection (screen), or the like.
By the way, when lamp voltage across an HID lamp is high, its lamp current decreases and the temperature of electrodes and within a bulb of the lamp decreases, and therefore active chemical characteristics within the bulb are suppressed. When such phenomenon occurs in a metal halide lamp, its halogen cycle does not get active. Usually, a protrusion is formed on the surface of the electrode, and the protrusion serves as an arc origin so that the arc origin is stabilized, but the protrusion is difficult to be formed on the surface of the electrode in a condition that the active chemical characteristics within the bulb are suppressed, as described above. On account of this, the arc origin is not fixed, and the phenomenon that the arc origin moves is easy to occur. Also, if a protrusion is not formed on the surface of the electrode and the arc origin does not stabilize, the entire electrode suffers damage by the arc, resulting in early electrode degradation.
Incidentally, a technology for reducing flicker of a discharge lamp is suggested in, for example, Japanese Patent National Publication number 2002-532866. In this technology, the shape of a lamp current through the lamp is modified in accordance with detection of flicker generation.
A technology suggested in Japanese Patent Application Publication number 2002-134287 gradually increases an instantaneous value of power supplied to a discharge lamp as time is passed in a half period of a lamp current through the lamp.
According to the technologies described in the Patent Application Publication number 2002-352982 and the Patent National Publication number 2002-532866, electrode wear is controlled. The latter especially directs its attention to reduction of flicker, but there is a need to superpose a pulse-shaped current in order to modify the shape of the lamp current through the lamp, so that a comparatively complicated control is required.
A technology described in the Patent Publication number 2002-134287 changes the instantaneous value of power supplied to the lamp and also changes each wave form of voltage applied across the lamp or the lamp current into wave form except square wave. Therefore a comparatively complicated control is required.